Dopant diffusion is a standard process that is used when fabricating some semiconductor-based devices. One important application for this process is to create buried p-n junctions, such as are formed during the manufacture of optoelectronic photodetectors.
To create buried p-n junctions via dopant diffusion, a diffusion mask is first patterned onto a wafer (e.g., indium phosphide wafer, etc.). Typically, the wafer is characterized by a background dopant concentration of either n-type (i.e., resulting in an n-type wafer) or p-type (i.e., resulting in a p-type wafer). The mask includes “windows” that are open to the underlying semiconductor at specific regions. The specific arrangement of the windows in the mask is based on the layout and geometry of the devices being formed.
Dopant atoms of the opposite type in the wafer (i.e., n or p) are delivered to the diffusion mask. The dopant atoms diffuse through the windows and then into the semiconductor to form p-n junctions. The diffusion process is controlled via parameters such as temperature and dopant partial pressure. Typically, elevated temperature (about 500° C.) is required to achieve reasonable diffusion times.
To form reliable, high performance, uniform buried p-n junctions requires a well-controlled diffusion process. To achieve this control, the following process capabilities are typically required:                The source of dopant material must be well controlled (e.g., good control over dopant partial pressure, etc.).        The process must be isolated from contaminants that can disrupt the diffusion process or adversely affect material quality.        
Several approaches are known for implementing the dopant diffusion process. Perhaps the simplest approach is “sealed ampoule diffusion,” wherein a wafer (e.g., indium phosphide wafer, etc.) is sealed in a clean vessel along with a source of dopant atoms. The entire vessel is heated to a required diffusion temperature to cause the dopant atoms to diffuse into the wafer.
More recently, dopant diffusion processes have been conducted in reactors that were initially designed for epitaxial growth. These so-called “epi-reactors” have proven to be an excellent environment for dopant diffusion. The reason for this is that the primary process requirements for epitaxial growth of semiconductors—namely, precise control of source material, highly-accurate temperature control, and a contamination-free process environment—are identical to those for dopant diffusion. Furthermore, the epi-reactor process provides a much finer degree of control over a variety of important process parameters (e.g., dopant precursor concentration, ambient environment during diffusion, ambient overpressure, etc.) than alternative processes.
The use of the epi-reactor for dopant diffusion has focused mainly on metal-organic chemical vapor deposition (MOCVD) for MOCVD diffusion. Researchers have used this platform for p-type doping of InP (and related materials such as InGaAs and InAsP) with zinc (Zn) using an appropriate precursor dopant source such as diethylzinc (DEZn) or dimethylzinc (DMZn).
To date, the primary focus of experimentation with dopant diffusion has been to achieve process control and reproducibility. Studies have considered the influence on the diffusion process of parameters such as: temperature; dopant partial pressure; diffusion time; the effect of annealing operations; the effect of different carrier gases; and the influence of dislocation density, among others.
To be sure, these parameters provide a measure of control over the process, but they do nothing to address certain limitations of the dopant diffusion process, as currently practiced. For example, thus far, dopant diffusion in an epi-reactor has been limited to providing a single diffusion depth for all devices that are processed during a single execution of the process.
It would be very desirable to be able vary, within a single execution of the process and from device-to-device, diffusion depth. But doing so will require the use of control parameters that are different from those that have been studied and utilized in the prior art.